A Complexity-Brightness Correlation in Gamma-Ray Bursts

ABSTRACT We observe strong correlations between the temporal properties of gamma-ray bursts (GRBs) and their apparent peak brightness. The strongest e †ect (with a signiÐcance level of D10~6) is the di †erence between the brightness distributions of simple bursts (dominated by a single smooth pulse) and complex bursts (consisting of overlapping pulses). The latter has a break at a peak Ñux of D1.5 photons cm~2 s~1, while the distribution of simple bursts is smooth down to the BATSE threshold. We also observe brightness-dependent variations in the shape of the average peak-aligned time proÐle (ATP) of GRBs. The decaying slope of the ATP shows time dilation when bright and dim bursts are compared, while the rising slope hardly changes. Both slopes of the ATP are deformed for weak bursts as compared to strong bursts. The interpretation of these e †ects is simple : a complex burst in which a number of independent pulses overlap in time appears to be intrinsically stronger than a simple burst. The BATSE sample of complex bursts then covers larger redshifts, where some cosmological factor causes the break in the peak brightness distribution. This break could correspond to the peak in the star formation rate that was recently shown to occur at a redshift of z D 1.5